The acid phosphatase found in the human prostate gland is antigenically unique. Therefore, this enzyme has been regarded as a marker antigen of the prostate, and various immunological assay methods have been developed for this enzyme in order to utilize this marker as a diagnostic tool in the evaluation of prostatic cancer. In order to elucidate the molecular basis of antigenic specificity, we have dissociated the enzyme into subunits and cleaved the subunits into peptide fragments by CNBr. We found that, in the presence of specific antibodies directed against human prostatic acid phosphatase, both subunit polypeptides and one of the CNBr-derived peptide fragments regain catalytic activity. Further work is in progress to elucidate the antigenic structure and amino acid sequence of the active site of this enzyme. We have introduced a simple immunoassay called immunoenzymes assay. In this assay, enzyme-antibody complexes were precipitated either by second antibody or by ammonium sulfate and the enzyme activity was determined. Clinical application of this assay method is in progress in this laboratory.